250+ TOP MCQs on 3-Phase Bridge Inverters-3 and Answers

Power Electronics Multiple Choice Questions on “3-Phase Bridge Inverters-3”.

1. The 120° mode of operation of a three phase bridge inverter requires ___________ number of steps.
A. 2
B. 4
C. 6
D. 8
Answer: c
Clarification: Like the 180 mode, the 120° mode also requires six steps, each of 60° duration.

2. In case of the 120° mode of operation, __________ devices conduct at a time.
A. 2
B. 3
C. 4
D. none of the mentioned
Answer: A
Clarification: Unlike the 180° mode, in the 120° mode one 2 devices are conducting at a time as each conduct for 120°.

3. Safe commutation can be achieved in case of the ____________ operating mode.
A. 180°
B. 120°
C. 360°
D. none of the mentioned
Answer: B
Clarification: In the 120 mode of operation, there is a 60° interval between turning off of T1 and turning on of T4 (or any two SCRs belonging to the same leg), hence SCRs are commutated safely.

4. What is the R phase voltage when only T1 and T2 are conducting from 60° to 120°. Consider star connected R load.

A. Vs
B. Vs/2
C. –Vs/2
D. 0
Answer: B
Clarification: As only 2 SCRs are conducting at a time, it is 120 mode.
When T1 and T2 are conducting, R phase is connected to the positive bus through the T1 SCR and B phase is connected to the negative bus through the T2 SCR.
Considering that the load is a balanced R load.
V_RN = Vs/2
V_YN = -Vs/2.

5. If T1 is gated at 0 °, T3 and T5 will start conducting at _______ and _________ respectively.
A. 180°, 270°
B. 120°, 240°
C. 180°, 300°
D. 240°, 360°